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List: linux-man
Subject: Re: [PATCH] madvise.2: Clarify addr/length and update hugetlb support
From: Mike Kravetz <mike.kravetz () oracle ! com>
Date: 2022-05-25 20:50:19
Message-ID: f8a5053c-f7e6-e64d-cc26-eb641dfd9215 () oracle ! com
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On 5/25/22 11:38, Peter Xu wrote:
> Hi, Mike,
>
> Some minor nitpicks below.
>
> On Tue, May 24, 2022 at 04:28:44PM -0700, Mike Kravetz wrote:
>> .B MADV_DONTNEED
>> -cannot be applied to locked pages, Huge TLB pages, or
>> +cannot be applied to locked pages, or
>> .BR VM_PFNMAP
>> pages.
>
> This looks good, but since this will be a behavior change and we won't be
> able to change the old kernels, I saw the man page normally does this with
> things like:
>
> Since Linux 5.18, this madvise supports hugetlbfs page>
> Majorly it states starting from which version it'll work, and when it'll
> not.
You are right. I will add this.
>
>> (Pages marked with the kernel-internal
>> @@ -170,24 +174,24 @@ Note that some of these operations change the semantics of memory accesses.
>> .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
>> Free up a given range of pages
>> and its associated backing store.
>> -This is equivalent to punching a hole in the corresponding byte
>> +This is equivalent to punching a hole in the corresponding
>> range of the backing store (see
>> .BR fallocate (2)).
>> Subsequent accesses in the specified address range will see
>> -bytes containing zero.
>> +pages containing zero.
>> .\" Databases want to use this feature to drop a section of their
>> .\" bufferpool (shared memory segments) - without writing back to
>> .\" disk/swap space. This feature is also useful for supporting
>> .\" hot-plug memory on UML.
>> .IP
>> The specified address range must be mapped shared and writable.
>> -This flag cannot be applied to locked pages, Huge TLB pages, or
>> +This flag cannot be applied to locked pages, or
>> .BR VM_PFNMAP
>> pages.
>> .IP
>> In the initial implementation, only
>> .BR tmpfs (5)
>> -was supported
>> +supported
>> .BR MADV_REMOVE ;
>> but since Linux 3.5,
>> .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
>> @@ -196,9 +200,9 @@ any filesystem which supports the
>> .BR FALLOC_FL_PUNCH_HOLE
>> mode also supports
>> .BR MADV_REMOVE .
>> -Hugetlbfs fails with the error
>> -.BR EINVAL
>> -and other filesystems fail with the error
>> +Filesystems which do not support
>> +.BR MADV_REMOVE
>> +fail with the error
>> .BR EOPNOTSUPP .
>> .TP
>> .BR MADV_DONTFORK " (since Linux 2.6.16)"
>> @@ -596,6 +600,18 @@ that are not mapped, the Linux version of
>> ignores them and applies the call to the rest (but returns
>> .B ENOMEM
>> from the system call, as it should).
>> +.PP
>> +If the specified address
>> +.I addr
>> +is within a mapping backed by Huge TLB pages, then
>> +.I addr
>> +must be aligned to the underlying Huge TLB page size. If the range
>> +specified by
>> +.I addr
>> +and
>> +.I length
>> +ends in a mapping backed by Huge TLB pages, then the end of the range
>> +will be rounded up to a multiple of the underlying Huge TLB page size.
>
> I'm slightly worried this could be hidden too deep, meanwhile it duplicates
> part of the sentence of how start/end will be treated.
Yes, I just dumped more stuff into the NOTES section. Will rearrange as
you suggested.
>
> How about adding a short paragraph into each of MADV_DONTNEED and
> MADV_REMOVE section (right after the new sentences upon hugetlbfs), with:
>
> For hugetlbfs, the start/end alignments on page sizes will be based on
> huge page size.
>
> No strong opinions on any of these. Anyway:
>
> Acked-by: Peter Xu <peterx@redhat.com>
Thanks Peter
--
Mike Kravetz
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